The present invention generally relates to digital cross connect systems, and more particularly to a digital cross connect system which cross connects VT signals of the synchronous optical network (SONET) with frame phase synchronization.
Recently, the SONET standards have been newly prescribed in the United States. Hence, there are demands to realize a digital cross connect system which can not only cross connect the line signals such as the conventional ASYNC-DS3, DS2, DS1C and DS1 but also cross connect signals such as OC-1 and OC-3 which are in conformance with the SONET standards. When the frame structure used for the signal processing in such a digital cross connect system is the SONET-VT format, there is a need for a signal processing method which achieves frame synchronization because no frame synchronizing signal is used in the SONET-VT format.
FIG.1 generally shows a conventional digital cross connect system which terminates only the line signals such as the conventional ASYNC-DS3, DS2, DS1C and DS1. The digital cross connect system includes reception interface parts 51, a switching part 52, transmission interface parts 53 and a control part 54. The reception interface part 51 receives the line signals such as the ASYNC-DS3, DS2, DS1C and DS1 and converts the line signals into frames. The switching part 52 cross connects output signals of the reception interface parts 51. The transmission interface part 53 converts the cross connected signal from the switching part 52 into a line signal and transmits the same. The control part 54 controls the switching part 52.
A frame format shown in FIG.2 is used for the signal processing within the system. In FIG. 2, .phi. denotes a time slot indicating an input #1 of the DS1 signal, F0 and F1 denote frame synchronizing signals, Ci denotes a stuff control signal and V1 denotes a time slot for stuffing.
When an attempt is made to terminate the 0C signal of the newly prescribed SONET with the conventional frame format used for the signal processing within the system, it is impossible to terminate this OC signal because the transmission capacity of the frame is insufficient to directly process the OC signal.
For this reason, in order to not only terminate the line signals such as the conventional ASYNC-DS3, DS2, DS1C and DS1 but also the OC signal of the SONET standards, a new frame format is required for the signal processing within the system and the SONET-VT format may be used as the new frame format.
By using of the VT format for the signal processing within the system, it becomes possible to cross connect the STS-1 signal or the DS3 signal and the VT1.5 signal inserted in the OC signal as they are without terminating these signals. In addition, when terminating the VT1.5 signal, there is an advantage in that the V1, V2, V3 and V4 bytes can be simply converted by replacing these bytes by frames.
However, no frame identification pattern (frame synchronizing pattern) is provided in the VT format. For this reason, when a phase error is introduced in the VT signals having the VT format on the channels output from the reception interface parts 51 due to a transmission delay within the system, it becomes impossible to carry out a normal signal processing in the switching part 52 or the transmission interface parts 53.
FIG.3 shows the frame format of the VT signal in conformance with the SONET standards. The VT frame includes four VT payload pointer parts V1, V2, V3 and V4, and the VT payload pointer parts V1, V2, V3 and V4 respectively have a data part having a bytes added thereto. The number of a bytes of the data part is a=26 in VT1.5, a=35 in VT2, a=53 in VT3 and a=107 in VT4.
For example, the data in the data part has a format shown in FIG.4 with 104 bytes in the case of VT1.5 frame. This data is divided into four and is distributed and arranged in each data part of the VT1.5 frame. In FIG.4, I denotes an information bit, O denotes an overhead bit, C denotes a stuff control bit, S denotes a stuff bit and R denotes a reserved bit (fixed stuff bit). The position of V5, that is, the distance between V2 and V5, is determined by contents of the V1 and V2 bytes.
As described above, the VT signal of the SONET standards does not have a frame synchronizing signal. For this reason, in order to cross connect the VT signal, it is necessary to add a frame synchronizing signal by some means. As one conceivable method, it is possible to add the frame synchronizing signal on the outside of the VT frame format as shown in FIG.5.
However, when the frame synchronizing signal is added to the VT frame as an additional byte, the signal frequency processed within the system becomes high. For example, in the case of the VT1.5 frame, the signal frequency is 1.728 Mbps, but the signal frequency becomes (1.728 M+A) bps when the frame synchronizing signal is added.
In addition, in order to add the frame synchronizing signal to the VT frame, it is necessary to temporarily store the VT frame and a memory is required for this storage. As a result, the need to temporarily store the VT frame not only enlarges the scale of the hardware but also introduces an undesirable signal delay caused by the writing and reading of the VT frame to and from the memory.
On the other hand, the transmission delay of each channel signal is inevitably caused by the difference in the lengths of transmission paths due to the positions of the transmission and reception interface parts of each of the channels when the digital cross connect system is assembled on a single frame or the positions of the transmission and reception interface parts of each of the channels when the digital cross connect system is distributively arranged within a single building, as shown in FIG.6. Hence, measures must be taken against this transmission delay.
As one measure, FIG.7 shows a conceivable system using a phase absorbing means 56. In other words, the switching part 52 supplies to all channel interface parts 55 a reference timing signal which is common to all channels. The channel interface part 55 functions both as a transmission interface part and a reception interface part. In addition, a phase absorbing means 56 is provided at the input/output side of the switching part 52.
According to this conceivable system shown in FIG.7, the reception part of each channel interface part 55 transmits the reception signal of its own channel in response to the reference timing signal which is output from the switching part 52. The signals of each of the channels have a phase error of one bit or more when input to the phase absorbing means 56 due to the difference in the lengths of transmission paths. However, the phase absorbing means 56 adjusts the phases of the signals for each of the channels so that the signals are input to the switching part 52 with the same phase.
Furthermore, the signal which is subjected to the cross connect process in the switching part 52 is transmitted to the transmission part of each interface part 55 similarly in response to the reference timing signal, and a phase error similar to the above also occurs. Hence, the phase absorbing means 56 matches the phases of the signals to the reference timing.
However, when the phase absorbing means 56 is used, it is necessary to adjust the setting of the phase absorbing means 56 for each system when starting or initializing the system because the conditions such as the set up position of the channel interface parts 55 differ for each individual system. As a result, the set up procedure of the system increases, and moreover, a skilled maintenance person is needed for the adjustment because the timing adjustment is delicate and requires skill. In addition, the provision of the phase absorbing means 56 enlarges the scale of the hardware of the system.